The goal of this study is to examine the validity and reliability of the LifeChek KAL-X Sensor to assess energy expenditure during varying modes and intensities of physical activity. We will also examine whether environmental conditions or variations in clothing impact the accuracy or performance of the KAL-X Sensor. In addition, we will examine the correlation between the KAL-X Sensor and self-reported physical activity across a period of free-living. This will also permit us to examine if participants are willing to wear this device for an extended period of time and the integrity of the sensor under free-living conditions. This application is in response to a recent program announcement by the NIH (PAR-03-009). The proposed study will involve three phases; in Phase 1 we will compare energy expenditure from the KAL-X Sensor to a criterion measure of energy expenditure (indirect calorimetry). This will involve the subjects performing a variety of activities including the following: treadmill walking, cycle ergometry, armcrank ergometry, resistance exercise, and a simulation of lifestyle activity. Each activity session will be performed on a separate day with energy expenditure estimated using the LifeChek KAL-X Sensor and the criterion measure of energy expenditure will be indirect calorimetry. To assess whether variations in bodyweight and body size impact the accuracy of the KAL-X Sensor, we will recruit equal numbers of men and women from three body mass index categories (20-24.9 kg/m 2, 25-29.9 kg/m 2, 30-35 kg/m2). Based on a power analysis, 48 subjects (16 in each body mass index category) will be recruited to participate in this study. In Phase 2 subjects will perform an outdoor activity for a period of approximately 60 minutes. Again, energy expenditure will be estimated using the LifeChek KAL-X Sensor and the criterion measure of energy expenditure will be indirect calorimetry. We will also monitor environmental conditions (temperature, humidity, and cloud cover) to assess whether these factors impact the accuracy of the KAL-X Sensor. In Phase 3 subjects will wear the KAL-X Sensor for a one-week period under free-living conditions. This will allow us to compare energy expenditure from the KAL-X Sensor with energy expenditure estimated from self-report using a questionnaire. Moreover, we will be able to examine if participants are willing to wear this device and the integrity of the KAL-X Sensor under free-living conditions. We hypothesize that there will be no significant difference between the measurements of energy expenditure in the validation study between the KAL-X Sensor and indirect calorimetry. In addition, we hypothesize that clothing and environmental conditions will not affect the accuracy or reliability of the KAL-X Sensor. Moreover, we hypothesize that subjects will be willing to wear the KAL-X Sensor and that the sensor will be reliable under free-living conditions. The KAL-X Sensor is an innovative technology that may revolutionize the assessment of energy expenditure and may have implications for improving outcomes in clinical programs.